January 19, 2005
Source: American Phytopathological Society, Plant Disease Notes
[edited]
Asian soybean rust: incidence, severity, and morphological
characterization of Phakopsora pachyrhizi (Uredinia and
Telia) in Argentina
M. A. Carmona and M. E. Gally, Facultad de Agronomia,
Universidad de Buenos Aires, Av. San Martin 4453 (1417), Buenos
Aires, Argentina; and S. E. Lopez, Departamento de Biodiversidad
y Biologia Experimental, Facultad de
Ciencias Exactas y Naturales. Universidad de Buenos Aires, Av.
San Martin 4453 (1417), Buenos Aires, Argentina. Plant Dis.
89:109, 2005; published on-line as DOI: 10.1094/PD-89-0109B,
2005. Accepted for publication 8 Oct 2004.
Asian soybean rust (ASR), caused by _Phakopsora pachyrhizi_, is
the most destructive disease of soybean (_Glycine max_) in many
areas of the world. ASR was 1st detected in Argentina during
2002 in a limited area in the northern region of the country
(2). During the 2004 growing season, _P.pachyrhizi_ spread
rapidly throughout most soybean growing areas of northwestern
and northeastern Argentina. ASR was also was found in some
fields in Entre Rios and Santa Fe provinces. In all areas,
symptoms were expressed late in the 2004 season (growth stages
R5.5 to R7), and yield losses were minimal.
The objectives of this study were to quantify _P. pachyrhizi_
infection in the canopy and morphologically characterize the
fungus from fields where it had been previously detected by
polymerase chain reaction (PCR)(3).
Incidence (percentage of plants affected) and severity
(percentage of leaf area affected, including chlorosis) were
visually estimated for 10 plants arbitrarily collected (April
2004) from each of 3 fields located in Charata (Chacabuco),
Chaco Province (Sample 1, collected in the R6 stage), La Paloma
(Moreno), Santiago del Estero Province (Sample 2, stage R6 to
R7), and Tolloche (Anta), Salta Province (Sample 3, stage R5.5).
Disease assessments were made for the lower, middle, and upper
canopy from 15 leaves per plant. The number of pustules per sq
cm and uredinia per lesion were recorded from the undersides of
central leaflets for each trifoliolate observed. Tissue sections
were made to observe fructifications of _P. pachyrhizi_.
Incidence of affected plants was 100 percent in all fields.
Disease severity for Sample 1 was 45 percent (range 30 to 60
percent), 20 percent (10 to 30 percent), and 10 percent (5 to 20
percent) for the lower, middle, and upper canopy, respectively;
for Sample 2: 60 percent (30 to 80 percent), 40 percent (25 to
50 percent), and 25 percent (15 to 40 percent) for the lower,
middle, and upper canopy, respectively; and for Sample 3: 25
percent (10 to 50 percent), 15 percent (10 to 20 percent), and
10 percent (5 to 15 percent) for the lower, middle, and upper
canopy, respectively.
The number of pustules per sq cm for Sample 1 was 156/sq cm
(range 88 to 200); Sample 2: 172/sq cm (128 to 232); and Sample
3: 120/sq. cm (72 to 232). The number of uredinia per lesion for
Sample 1 was 6 per lesion (range 1 to 15); Sample 2: 5.5 per
lesion (1 to 13), and Sample 3: 2.8 per lesion (1 to 5).
The 2 spore types that were commonly observed were
urediniospores and teliospores. Telia were found on infected
leaves mixed with uredinia in every sample. Urediniospores
measured 16 to 22 micrometers (mean 18.5 micrometers) x 25 to 30
micrometers (mean 27 micrometers). Teliospores measured 8 to 11
micrometers (mean 9 micrometers) x 19 to 27 micrometers (mean
23.8 micrometers). Spores sizes are in the range described by
Ono et al. (1).
To our knowledge, this is the 1st report of epidemiological and
morphological characterization of ASR in Argentina and the 1st
report of the telial stage of _P. pachyrhizi_ on soybean in
South America.
References:
(1) Y. Ono et al. Mycol. Res. 96:825, 1992.
(2) R. L. Rossi. Plant Dis 87:102, 2003.
(3) SINAVIMO, Sistema Nacional Argentino de Vigilancia y
Monitoreo de plagas. Roya de la soja: Resultados de la campana
2003-2004. On-line publication. SENASA, 2004.
--
ProMED-mail
<promed@promedmail.org>
[The report that ASR is present in Argentina is significant, but
not unexpected. Of more interest is that ASR, now that it is
established on kudzu and perhaps other leguminous hosts in the
southern tier of USA states, can be expected to spread northward
by wind from these hosts, thus expanding the range of ASR in the
country. Whether ASR teliospores survive winter conditions in
these states remains to be determined. If ASR is established on
susceptible hosts and teliospores remain viable, these hosts
will almost certainly serve as potent inoculum sources for
further spread northward. ASR, now that it is established in the
USA, would be expected to cause significant crop losses and
perturbations in commercial trade. ASR is present in Asia:
Russia Far East, Korea, Japan, China, Taiwan, Philippines,
Nepal, India; Australia; Africa: Nigeria, Mozambique, Rwanda,
Uganda, South
Africa, Zambia, Zimbabwe; South America: Brazil, Paraguay,
Argentina; Oceania and the USA (several southern states).
Links:
<http://bldg6.arsusda.gov/~pooley/soy/cooper.html>
<http://www.aphis.usda.gov/lpa/issues/sbr/sbr.html>
<http://www.aphis.usda.gov/ppq/ep/soybean_rust/UreMelPp502.pdf>
- Mod.DH]
